Why could cooling setpoint could not be maintained?

llyces1985 · Posts: 32

Hi all,

In the results of my simulation, I found that the ‘air T’ could not maintain at ‘ cooling setpoint’. The reason maybe the palnt sizing lacks power! In the model option, I set the ‘HVAC sizing’ as ‘ Autosize’ type. And in the help file, it says that “When using Compact HVAC the components and flow rates are sized using the EnergyPlus autosizing mechanism to ensure that the required comfort conditions are met” about the ‘ Autosize’ type. However, the ‘cooling setpoint’ was not met! I tried to make the ‘air supplied T’ lower than 12°C and choose a higher CoP value. The ‘air T’ could not maintain at ‘cooling setpoint’ as I desire to. So, how to analyze and work out such problem? Any idea provided would be appreciated! Thanks a lot!
All the best,

Esong

Laurent_Support · Posts: 292 Location: Vincennes

Hello Esong,

If you don't want any power limitation, use the Adequate Plant sizing option.

Another thing, if you ask for Operative temperature setpoint, the air supplied into the zone could need to really lesser than expected if radiant temperature is high.

Laurent

llyces1985 · Posts: 32

Hi Laurant,

I’m confused again! Several days ago, the ‘cooling setpoint’ could not be maitained and the ‘air T’ was higher than the ‘cooling setpoint’ which seems that the ‘plant sizing’ lacks power. Then I choosed the ‘Adequate sizing’ option, simulated again, I found that the ‘air T’ is lower than the ‘cooling setpoint’. Then I choosed the ‘Autosize’ option again, and the ‘air T’ was lower than the ‘cooling setpoint’ again. Why? I changed the ‘supplied air T’ more higher than before and it was useless! Is there any wrong in my model? So could I take out your some time and ask for you help me to check it please( my model is .skh type even though I choose .dsb type in every save option, so I compress it in .rar type)? If you find that there’s abnormity or mistakes, please let me know! Waiting with anticipation for your comments! Thank you very much in advance!
All the best,

Esong

Laurent_Support · Posts: 292 Location: Vincennes

Hello Esong,

The plant sizing option is not the problem here, this is why changing it or temperature setpoints has no consequences.

In your building, you're using a VAV AHU for some zones. In this zone you ask for a Min fresh air/pers so system provides the zone with a minimum ac/h.
VAV supplies many zone and MUST provides a air temperature low enough to fight heat in the hottest zone. This will set the air temperature blown into the building.
Other zones which do not have so much cooling demand are also supplied with this low air temperature and for a min ac/h ... that could bring more cool air than needed and explains why internal temperature are under cooling setpoint.

If you look to the zones where you set a Fan coil or unitary system you will notice setpoint are easily manage.

Best regards,
Laurent

llyces1985 · Posts: 32

Hi Laurent,

I maybe understand what you say. However, I changed the ac/h in the HVAC template data/Ventilation/Mechanical Ventilation option. I defined a lower value of 2 than the former one(3). And I choosed ‘2-Min fresh air(per person)’ in the ‘Outsied air definition method’ tab. And after simulation, I also found that the ‘air T’ could not be maitained at ‘cooling setpoint’ either. And in such situation, I found that the ‘radiant T’ value was very higher and the ‘Fanger PMV’ value was over 3.5 in the ‘play ground’ zone! This results come out in the vav system zones, but not FCU zones.So it maybe not the real solution! You advised me to look to the ‘FCU’ zones and I followed. The results were that the ac/h of vav system was same as the fcu and the ‘Outsied air definition method’ too! So, why the ‘air T’ can be maintaied at ‘cooling setpoint’ in fcu zones but in vav zones can not? I know that, just as you say “Other zones which do not have so much cooling demand are also supplied with this low air temperature and for a min ac/h ... that could bring more cool air than needed and explains why internal temperature are under cooling setpoint.” After my efforts, I failed to get the desired results! I think that there must be sth I ignored and not understand completely! So I hope that you could give me some comments again! Sorry to take out your time again! Thanks in advance!
All the best,

Esong

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